I have a VHT pittbull UL, that uses 4 KT88's. I had 4 new tubes installed about 3 months ago(Sovtek, all 4 matched) by a local professional luthier.

Normally, all 4 of my tubes have a nice subtle blue glow when standby is off.

I noticed recently that one of the tubes does not have that blue glow anymore when standby is off - I trued moving the tubes around, and sure enough, the same tube does not glow, but the others do.

I contacted the dealer I got them from (tubestore), and they said it's nothing to worry about unless I noticed a big difference in sound, which I'm fairly sure there is no difference. I'm wondering if I should return the tubes for testing/replacement under warranty just in case.

It is fine. The inside of the bulb was probably cleaner than the others. A bright blue glow often means the input grid capacitor is leaking DC and should be replaced. The lack of glow could mean low or no emission and that you could notice.

I noticed recently that one of the tubes does not have that blue glow anymore when standby is off - I trued moving the tubes around, and sure enough, the same tube does not glow, but the others do.

This blue glow is caused by cobalt contamination. The cobalt ions fluoresce when hit by stray electrons. Seen the same thing myself. The RCA 807s glowed prettily when they were new. Over time, the glow has lessened considerably. It's only an aesthetic detriment.

Other RCA tubes (6AQ5s) also started out with that blue glow that has almost completely disappeared.

So the inside somehow got cleaner is what you are suggesting? Like I said, the last 2.8 months the tubes have all had the exact same glow, now suddenly, one does not, regardless of where I move it...?

On just about any material produced the start and end of the run are a bit different from the middle of the batch. Even when formed in laddles the sides of the product are different. So one tube stopped glowing first, only a cosmetic issue.

Alright I see that it is normal for this to happen. But why just this one tube? why suddenly? One day, it had the exact same glow, the next, no glow at all. I did even more 'cosmetic' testing and had the amp in standby in a pitch black room(night time no lights on) and took standby off, and the other three tubes all glowed nicely, while the problem tube still had 0% blue glow. It just seems odd that tubes that normally glow blue, just suddenly stop.

To me, no one has explained why it stopped glowing blue, only showing links and explaining WHY they glow blue(or dont). Don't get me wrong, these are all great answers, but still don't answer my question. Thanks for the help!

Alright I see that it is normal for this to happen. But why just this one tube? why suddenly? One day, it had the exact same glow, the next, no glow at all. I did even more 'cosmetic' testing and had the amp in standby in a pitch black room(night time no lights on) and took standby off, and the other three tubes all glowed nicely, while the problem tube still had 0% blue glow. It just seems odd that tubes that normally glow blue, just suddenly stop.

To me, no one has explained why it stopped glowing blue, only showing links and explaining WHY they glow blue(or dont). Don't get me wrong, these are all great answers, but still don't answer my question. Thanks for the help!

That is a really good question, but since people don't even agree on exactly what causes the blue glow other than energy hitting substances it seems an accurate answer would require real work.

If however you want a soothing inaccurate answer it is because there was less contamination and the gettter finally absorbed all of it. Or the tube was bumped slightly and fewer electrons escape or the cathode emission fell off just enough to stop making the glow.